Understanding the Molecular Mechanisms of Succinic Semialdehyde Dehydrogenase Deficiency (SSADHD): Towards the Development of SSADH-Targeted Medicine.

Succinic semialdehyde dehydrogenase deficiency (SSADHD) is a rare genetic disorder caused by inefficient metabolic breakdown of the major inhibitory neurotransmitter, γ-aminobutyric acid (GABA). Pathologic brain accumulation of GABA and γ-hydroxybutyrate (GHB), a neuroactive by-product of GABA catabolism, leads to a multitude of molecular abnormalities beginning in early life, culminating in multifaceted clinical presentations including delayed psychomotor development, intellectual disability, hypotonia, and ataxia. Paradoxically, over half of patients with SSADHD also develop epilepsy and face a significant risk of sudden unexpected death in epilepsy (SUDEP). Here, we review some of the relevant molecular mechanisms through which impaired synaptic inhibition, astrocytic malfunctions and myelin defects might contribute to the complex SSADHD phenotype. We also discuss the gaps in knowledge that need to be addressed for the implementation of successful gene and enzyme replacement SSADHD therapies. We conclude with a description of a novel SSADHD mouse model that enables 'on-demand' SSADH restoration, allowing proof-of-concept studies to fine-tune SSADH restoration in preparation for eventual human trials.

Intermittent Ethanol Access Increases Sensitivity to Social Defeat Stress.

BACKGROUND: Comorbidity between alcoholism and depression is extremely common. Recent evidence supports a relationship between alcohol exposure and stress sensitivity, an underlying factor in the development of depression. Our laboratory has recently shown that chronic alcohol gavage increases sensitivity to social defeat stress (SDS). However, the effects of voluntary alcohol consumption, resulting from protocols such as intermittent ethanol access (IEA), on defeat stress sensitivity have yet to be elucidated. METHODS: We first assessed the effects of 4 weeks of IEA to 20% alcohol on sensitivity to subthreshold SDS exposure. Next, to examine neuroinflammatory mechanisms, we analyzed gene expression of inhibitor of NFkB (IkB) following IEA or chronic alcohol exposure (10 days of 3.0 g/kg alcohol via intragastric gavage). Then, we quantified NFkB activation via ß-galactosidase immunohistochemistry following IEA or chronic alcohol gavage in NFkB-LacZ mice. RESULTS: IEA-exposed mice displayed an increase in sensitivity to subthreshold SDS compared to water-drinking controls. We also found that IkB gene expression was decreased in the nucleus accumbens (NAC) and amygdala (AMY) following IEA but was not altered following chronic alcohol gavage. Finally, we observed increased NFkB activity in the central amygdala (CEA), basolateral amygdala (BLA), and medial amygdala (MEA) after IEA, and increased NFkB activity solely in the CEA following chronic alcohol gavage. CONCLUSIONS: These findings further corroborate that prior alcohol exposure, in this case intermittent voluntary consumption, can impact development of depressive-like behavior by altering stress sensitivity. Furthermore, our results suggest the CEA as a potential mediator of alcohol's effects on stress sensitivity, as NFkB was activated in this region following both IEA and chronic alcohol gavage. Thus, this study provides novel insight on alterations in the NFkB pathway and identifies specific regions to target in future experiments assessing the functional role of NFkB in these processes.

Clinical and molecular outcomes from the 5-Year natural history study of SSADH Deficiency, a model metabolic neurodevelopmental disorder.

BACKGROUND: Succinic semialdehyde dehydrogenase deficiency (SSADHD) represents a model neurometabolic disease at the fulcrum of translational research within the Boston Children's Hospital Intellectual and Developmental Disabilities Research Centers (IDDRC), including the NIH-sponsored natural history study of clinical, neurophysiological, neuroimaging, and molecular markers, patient-derived induced pluripotent stem cells (iPSC) characterization, and development of a murine model for tightly regulated, cell-specific gene therapy. METHODS: SSADHD subjects underwent clinical evaluations, neuropsychological assessments, biochemical quantification of γ-aminobutyrate (GABA) and related metabolites, electroencephalography (standard and high density), magnetoencephalography, transcranial magnetic stimulation, magnetic resonance imaging and spectroscopy, and genetic tests. This was parallel to laboratory molecular investigations of in vitro GABAergic neurons derived from induced human pluripotent stem cells (hiPSCs) of SSADHD subjects and biochemical analyses performed on a versatile murine model that uses an inducible and reversible rescue strategy allowing on-demand and cell-specific gene therapy. RESULTS: The 62 SSADHD subjects [53% females, median (IQR) age of 9.6 (5.4-14.5) years] included in the study had a reported symptom onset at ∼ 6 months and were diagnosed at a median age of 4 years. Language developmental delays were more prominent than motor. Autism, epilepsy, movement disorders, sleep disturbances, and various psychiatric behaviors constituted the core of the disorder's clinical phenotype. Lower clinical severity scores, indicating worst severity, coincided with older age (R= -0.302, p = 0.03), as well as age-adjusted lower values of plasma γ-aminobutyrate (GABA) (R = 0.337, p = 0.02) and γ-hydroxybutyrate (GHB) (R = 0.360, p = 0.05). While epilepsy and psychiatric behaviors increase in severity with age, communication abilities and motor function tend to improve. iPSCs, which were differentiated into GABAergic neurons, represent the first in vitro neuronal model of SSADHD and express the neuronal marker microtubule-associated protein 2 (MAP2), as well as GABA. GABA-metabolism in induced GABAergic neurons could be reversed using CRISPR correction of the pathogenic variants or mRNA transfection and SSADHD iPSCs were associated with excessive glutamatergic activity and related synaptic excitation. CONCLUSIONS: Findings from the SSADHD Natural History Study converge with iPSC and animal model work focused on a common disorder within our IDDRC, deepening our knowledge of the pathophysiology and longitudinal clinical course of a complex neurodevelopmental disorder. This further enables the identification of biomarkers and changes throughout development that will be essential for upcoming targeted trials of enzyme replacement and gene therapy.